Before adding a subdomain, there are a number of important features to consider.
It is necessary to confirm the webmaster's rights for each subdomain in Yandex and Google.
The root domain and each subdomain must have at least 200 pages. Otherwise, search algorithms will recognize the subdomain as a regular second-level page.
If you do interlinking, then within a separate subdomain. You can also put links to the root resource, but it is better to place them in the footer, for example, link to the "About the company" page.
Subdomain names cannot exceed 63 characters, and there cannot be more than 127 levels.
It is better to add subdomains to the webmaster el salvador cell phone number list panel separately. This will facilitate personal monitoring and analytics in the future.
Each subdomain requires a separate profile, since the link mass of the root resource does not extend to subdomains.
The semantic cores by which the root site and subdomain are promoted must be different. One semantic core for all promoted resources can lead to their mutual competition and even to filters from search engines.
The content on the main resource and subdomains should be different so that search engines do not consider the pages as duplicates. If you need to register one text fragment on several subdomains, put the rel="canonical" meta tag, which will point to the canonical page. If you need, for example, to make product cards unique. Suitable options include indicating the region or city in the title, posting reviews indicating the region, differences in prices depending on the location, etc.
The root site and subdomains should have similar (but not identical) design.
Also, before making a subdomain, you need to be prepared for the fact that you may have to expand your staff for a separate subdomain. After all, each new subdomain means regular portions of content, separate technical support, and much more.